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Tektronix 661
Sampling scope
Tektronix 661

Produced from 1961 to (?)

Manuals – Specifications – Links – Pictures

The Tektronix 661 is a sampling oscilloscope that was introduced in 1961. It accepts two plug-ins, a sampling unit and a timing unit.

Key Specifications

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The 661 project was led by Norm Winningstad. In "Winning with People: The First 40 Years of Tektronix", Marshall Lee writes:

During the early stages of the transition from tubes to transistors, however, computer research focused on emitter-coupled logic circuits, or ECL circuits, which proved much faster than conventional circuits, but also demanded higher-speed measuring instruments, which meant a much wider bandwidth. In response to ECL research, by 1962 Winningstad's group had produced the Type 661 sampling oscilloscope, which provided the answer to high speed and screen resolution.

In addition to Winningstad, Chuck Edgar and George Frye worked on the 661 design.


Four sampling units were made:

  • 4S1 (0.35 ns rise time)
  • 4S2 (0.1 ns rise time)
  • 4S2A (90 ps rise time)
  • 4S3 (uses P6038 sampling probes)

Three timing units were made:

There is also a set of calibration fixtures, 067-0066-00, consisting of one Vertical Test Load Unit and one Sweep Test Load Unit plug-in, but these were likely internal-only products for use in production and service as the 067-0066-00 manual says that 067-0066-00 Test Load Units are not required for calibration of 661s in the field.

Inter-module signals

The timing units use tunnel diode triggering. Two 50 Ω coaxial cables in the scope connect the sampling unit to the timing unit. One of these cables sends the "internal trigger signal" from the sampling unit to the timing unit. The other cable sends the "start sample signal" from the timing unit to the sampling unit, telling it when to sample.

The connectors used for the inter-module coaxial connections were made by Gremar. The connector on the plug-in side is a Gremar 8212A. The connector on the scope side is a Gremar 8205A. When operating one or both of the plug-ins outside of the 661, the timing unit can be fed an external trigger signal via the timing unit's front panel. However, the sampling unit still needs the start sample signal from the timing unit. The Gremar extension cable, part number 012-070 (shown below) enables that.

Triggering modes

A 661 can be triggered in at least four distinct modes:

  • The 4S1 uses a trigger pickoff transformer to produce the internal trigger signal that can trigger the timing unit. The 4S1 is the only 661 sampling unit that produces an internal trigger signal.
  • An external trigger signal can be fed to the timing unit via its front panel.
  • The timing unit can be operated in in free-running mode and the resulting pulse signal can be the stimulus for the device under test. This mode is similar to a TDR.
  • The calibration signal generator in the 661 can be used as trigger source, as described below.

Subsystems of the 661

Other than the two plug-ins, the 661 mainframe essentially consists of four subsystems:

  • power supply
  • indicator
  • amplitude/time calibration signal generator
  • delayed pulse generator

Power Supply

The power supply is typical of Tektronix scopes of early 1960s. It is linear. All power rectifiers are silicon diodes. An OG3 tube is used as a voltage reference for the +300 V supply. The other supply voltages use the +300 V supply as their reference. The +19 V and -19 V supplies use BJT-based regulators. The other regulators are tube-based. A 45 second delay tube is used so that plate voltage isn't applied to any tube in the 661 until the cathodes are hot. The 661 has a 137°F/58 °C thermal cutoff. In practice, it doesn't run hot.


The indicator is a conventional X-Y indicator. The total CRT accelerating voltage is 3 kV. The 661 uses a T5030 CRT with P2 phosphor. The vertical and horizontal amplifiers are essentially the same, each consisting of a two-stage differential amplifier. The first differential stage is made of a pair of OC170 germanium PNP bipolar junction transistors driven single-endedly with emitters connected directly together (maximum voltage gain). The second differential stage is made of both triodes of a 6DJ8 tube with cathodes connected directly together (maximum voltage gain). The vertical and horizontal amplifiers have feedback loops around them that determine their gain.


The amplitude/time calibrator is a Colpitts oscillator that uses a 7119 tube. It produces clippped sine waves at frequencies from 100 kHz to 100 MHz and amplitudes from 1 mV to 1 V. The output is 50 Ω GR-874 connector. The signal from the calibration generator is available on the front panel and is also sent to the timing generator through the multi-pin plug-in connector. This allows the timing plug-ins to select "CAL" as a trigger source. In this mode, the calibration generator can be used as the stimulus for the device under test. In many situations, this eliminates the need for external triggering.

Delayed Pulse Generator

The delayed pulse generator is a tunnel diode circuit that produces a negative-going 250 mV pulse with a risetime of about 150 ps and a pulse width of about 400 ns. The output is a 50 Ω GR-874 connector. When a timing unit (e.g., a 5T1) triggers, it sends a pulse through pin 10 of the J4 interconnect to the delayed pulse generator, which regenerates the pulse. There are three versions of the 661 delayed pulse generator (serial numbers 101 through 2829, 2830 through 3459, 3460 and up). All three versions use a 50 mA, 6 pF germanium tunnel diode to generate the actual output pulse. In early 661 production, a 1N3130 tunnel diode was used. Then it was replaced by a TD1081. The circuit versions also differ in how they bias and trip the output tunnel diode.

Based on the available schematics, the 661 appears to have been designed in 1961.

During what years was it manufactured?
Why is it that the 661 has a dedicated high-speed coaxial interface between the sampling unit the timing unit

while later 560-series sampling systems (3S2, 3T77A, etc.) simply use the regular plug-in connector and mainframe wiring harness for routing trigger and timing signals between the two units?

Some 661s have a 41 pin Bendix connector, J5, Tek part number 131-212, on the rear panel, perhaps to allow the 661 to be interfaced to low speed data acquisition equipment or a computer. This is essentially a pass-through from J2 & J3, the secondary multi-pin connectors on the vertical & horizontal plug-ins. These connectors carry switch position information (number, magnitude & units) and clock & gate pulses. Not all plug-ins had this 2nd connector; the 5T1A does, while the 5T3 does not.

The Tektronix 012-064 is a plug-in extension cable for the 661.


The 661 is constructed similarly to late-model 500-series scopes. The chassis is made of sheet aluminum. Most wiring is on ceramic strips. The side panels come off like those of a 545B or 547. The plug-ins of a 661 are incompatible with any other Tek scope, but the construction style is similar. The 661, like the 500-series scopes, uses Amphenol 26-series connectors for the electrical interface between plug-in and mainframe, but the 661 uses a 24-pin version whereas the 500-series uses a 16-pin version of the connector. The sides and rear of the 661 painted identically to late-model 500-series scopes, i.e., Tek-blue wrinkle.



Some Plugins compatible with 661

Page Manufacturer Model Description Introduced Discontinued
067-0066-00 Tektronix 067-0066-00 Calibration Fixture 1961 (?)
4S1 Tektronix 4S1 Dual channel sampling plugin 1962 (?)
4S2 Tektronix 4S2 Dual channel sampling plugin 1962 (?)
4S2A Tektronix 4S2A Dual channel sampling plugin 1965 (?)
4S3 Tektronix 4S3 1 GHz dual-trace sampling plugin 1963 (?)
5T1 Tektronix 5T1 Sampling sweep unit 1961 (?)
5T1A Tektronix 5T1A Timing plugin 1963 (?)
5T3 Tektronix 5T3 Timing plugin 1965 (?)


Some Parts Used in the 661

Part Part Number(s) Class Description Used in
0G3 154-0291-00 Gas Discharge Tube (Voltage regulator) 85 V voltage reference 132 506 547 560 561 561A 561S 564 565 567 661 TU-4 Z
1N3130 152-0078-00 Discrete component 50 mA, 25 pF Germanium tunnel diode 661 281
1N3719 152-0182-00 Discrete component 10 mA, 50 pF germanium tunnel diode 422 661 7B92 7B92A
6DJ8 154-0187-00 154-0305-00 Vacuum Tube (Dual Triode) dual triode 132 161 310A 316 317 502 502A 503 504 506 515 516 519 526 529 RM529 533 535 536 543 544 545 545A 545B 546 547 549 555 556 561A 561S 564 565 567 581 581A 585 585A 661 1A4 1S1 60 2A60 63 2A63 67 2B67 3A1 3A1S 3A2 3A3 3A6 3A7 72 3A72 75 3A75 4S2 51 3B1 3B1S 3B2 3B3 3B4 3M1 3S76 3T77 3T77A 9A1 9A2 1121 80 81 82 86 B O W Z Telequipment D56 Telequipment S32A Telequipment D52 S-311 Telequipment TD51 Telequipment S52 Telequipment S51 Telequipment Type A TU-4
7119 154-0340-00 Vacuum Tube (Dual Triode) dual triode 067-0532-00 191 3A3 3B4 3B5 516 545B 549 661 Chemtrix 205
OC170 Discrete component alloy-diffused Germanium PNP transistor 321 661
SMTD907 152-0275-00 Discrete component 50 mA, 5 pF germanium tunnel diode 280 661
STD916 152-0098-00 Discrete component 10 mA, 90 pF tunnel diode 556 565 RM565 661
T5030 154-0264-00 154-0265-00 154-0266-00 154-0267-00 154-0341-00 CRT CRT 503 504 560 561 661
TD1081 152-0099-00 152-0334-00 Discrete component 50 mA, 6 pF germanium tunnel diode 280 661 017-0086-00